The present invention relates to an image processing apparatus for performing image processing such as quantization of an image signal input to a digital copying machine or a facsimile system, and outputting the processed image signal.
Conventional copying machines and facsimile systems aim at simply copying and transmitting an original with good reproductivity.
It is also known to represent the density of an original image by an electrical binary signal.
In order to change a density of an image to be copied, a slice level serving as a binary reference for the analog image signal is changed according to an instruction from an operator. As shown in FIG. 1, when an operator adjusts a density control mechanism to cause it to set an output image level at a high density level, a low slice level TL2 is used to digitize an image signal IS. However, if the output image level is set at a low density level, a high slice level TL1 is used. As shown by waveforms (2) and (3) in FIG. 1, a binary black level output signal has a narrow duration when the high slice level TL1 is used, as compared with the case wherein the low slice level TL2 is used. As a result, the density of the output image is decreased.
Along with the use of digital image signals and the development of digital processing, negative/positive (NEG/POS) reversal can be achieved by inverting the binary image signal. With NEG/POS reversal, a positive original image is converted to a negative image and vice versa, and the converted image is output.
Conventional slice level setting for NEG/POS conversion poses the following problem.
As shown in FIG. 2, if an output (2) obtained by inverting the binary image output according to the use of the high slice level TL1 when the density control mechanism causes the output signal level to be set at the low density level is compared with an output (3) obtained by inverting the binary image output according to the use of the low slice level TL2 when the density control mechanism causes the output signal level to be set at the high density level, the binary black level output signal (2) has a wider duration than that of the output (3). Therefore, an output of a high density appears as the output signal (2), even though the operator desires a low image density.
If identical setting is performed with and without NEG/POS reversal, the density output desired by the operator cannot be obtained. Therefore, the operator must perform density setting in accordance with the operation mode.